Frequency-Resolved Photoelectron Spectroscopy of Nucleobase-Water Anion Clusters

Abstract

Frequency-resolved photoelectron spectroscopy enables anion resonance states to be accessed and probed. In the first instance, we applied this technique to the atmospherically abundant pyruvate anion, which was observed to exhibit ultrafast dissociation upon photoexcitation with UV light. Our focus then shifted to study the anion resonance states of nucleobases, which are postulated to play a crucial role in low-energy electron attachment to DNA, inducing strand breakages and mutagenesis. Clusters of the uracil anion, U−, with weakly solvating molecules (Ar and N2) were investigated, shedding light on how the anion resonance states can be incrementally stabilised with respect to the neutral species, and offering the most accurate determination of the valence electron affinity of U to date. Uracil water cluster anions, U−(H2O)n=1−35 were studied, yielding n-dependent electron binding energies and resonance photoexcitation energies that were extrapolated to the bulk aqueous limit. We showed that each of the three lowest-lying π* resonances of U−(aq) become bound states, but with accountment for the anion-to-neutral reorganisation energy (within a linear response model), the upper two remain accessible via low-energy electron attachment to U(aq). The thymine nucleobase exhibited similar behaviour. Altogether, our results connect the known anion resonance energies of the isolated nucleobases with a condensed-phase picture that is better representative of the native DNA environment, offering insight into which anion resonances may participate in the electron-induced DNA damage mechanism. Finally, we studied kinetically trapped non valence states of U−(H2O)n, bearing resemblance to water cluster anions, (H2O)n−. Multiple isomers were observed with different electron binding energies, and the structures thereof were assigned with the aid of computations. The non-valence electron was found to shift further from the U molecule with increasing hydration, analogous to a diffusion-controlled dissociation process of a molecule-electron contact pair into a hydrated electron.